The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the in vivo measurement of the effect of thermal treatment on tissue.
MR-guided interventional procedures employ an MRI system to monitor the medical treatment procedure. Such medical treatment procedures include MR-guided biopsies, hyperthermia, cyroablation, and thermal ablation using laser, radiofrequency, and focused ultrasound. Images produced by the MRI system depict the anatomy being treated and the medical instrument used to perform the treatment. These anatomic images are produced in real-time as treatment is performed and they assist the physician in physically guiding the instrument into proper position.
With medical procedures that thermally treat or ablate tissues, the MRI system can also produce images that indicate the temperature of tissues, within their field of view. Such temperature maps indicate the location of heating and the amount of heating that has occurred in the target tissues and surrounding tissues. The physician uses this information to properly direct the heat produced by the medical instrument and to terminate the treatment at the proper moment. An exemplary MRI temperature mapping method is disclosed in U.S. Pat. No. 5,711,300.
Magnetic resonance guided high intensity focused ultrasound (FUS) ablation is an attractive non-invasive method to selectively ablate deep-lying tissue where bone and air are not present in the ultrasound acoustic path. The therapeutic value of this alternative surgical technique depends on the accuracy of the MR temperature imaging method. There are several MR imaging techniques for measuring temperature change: T1-weighted imaging, diffusion imaging, and water proton resonance frequency (WPRF) imaging. Among these different temperature-imaging methods, WPRF temperature imaging is the preferred technique for FUS ablation therapy. In WPRF temperature imaging, temperature changes are calculated from phase difference images, usually acquired using a fast gradient echo sequence performed before and after FUS sonication. Several studies have demonstrated that WPRF temperature imaging can be used to accurately monitor temperature change in in vivo muscle tissue during FUS ablation treatment.
While in general the WPRF temperature imaging technique has many advantages over other methods, it has limitations in certain clinical applications. The WPRF technique measures temperature change by sensing the temperature-dependency of the hydrogen proton in water molecules. The resonance frequency of hydrogen protons in fat tissue, however, does not exhibit the same temperature sensitivity as in water. Therefore, in anatomical regions with low water content but high fat content (e.g. the human breast) WPRF cannot reliably provide adequate temperature monitoring for FUS treatment. In fact, this limitation has posed great difficulty for MR guided FUS ablation for breast cancer treatment. While WPRF imaging may be successfully used to monitor temperature change in a tumor, the WPRF technique does not indicate temperature increase in surrounding fat. The practical result is that it is very difficult to aim the FUS ablation device because the resulting temperature rise at its focal point is not apparent when the tissues are primarily fat and the tumor(s) is very small.
Another limitation of the WPRF temperature imaging method is that the measured temperature is a relative temperature change which occurs during two independent image acquisitions. These acquisitions are usually performed before and after FUS sonication. The tissue temperature is then inferred from this temperature change assuming that the base temperature, which is acquired before FUS sonication, is a constant temperature throughout the ablation procedure. This assumption, however, is not true during the course of an ablation treatment, especially at the later part of the treatment when the tissue temperature in the surrounding volume of the tumor has been elevated due to FUS near field heat accumulation. Consequently, temperature measured from WPRF imaging may underestimate the actual temperature in tissue/tumor, and this potentially may cause injury to the surrounding healthy tissues.